Cathodic protection of metal containers for liquids



June 21, 1960 w. L. MILLER EI'AL 2,941,935

CATHODIC PROTECTION OF METAL. CONTAINERS FOR LIQUIDS Filed Oct. 31. 1958T'xal 'Tacrib.

S. Prei ser 5:311; Tqdor 20 gY ATTO N United States Patent CATHGDICPRQTECTEON OF METAL C-GNTAINERS FQR LIQUIDS Walter L. Miller, 16%)Hendrickson Ave, Lynbrook, N. Herman S. Preiser, 6904 Leesville Blvd, N.Springfield, Va.; and Sidney Tudor, 102-47 64th Road, Forest Hills N.Y.

Filed Oct. 31, 1958, Ser. No. 771,169

18 Claims. C1. 204-1%) (Granted under Title 35, US. Code (1952), see.266) The invention described herein may be manufactured and used by orfor the Government of the United States of America for governmentalpurposes without the payment of any royalties thereon or therefor.

This invention relates to the cathodic protection of relatively largemetal containers or tanks, for liquids to resist corrosion thereof duein part to galvanic currents created in the container or tank whenliquids are placed therein. This corrosion protection is accomplished byplacing in the metallic tank either at least one sacrificial galvanicanode, such as of magnesium, or at least one relatively inert anodeactivated by an impressed current. Cathodic protection of this type isvery important for cargo tanks that are often used as ballast tanks ontanker ships, as well as for other relatively large tanks or containersof water and other liquids. Cargo tanks of tanker ships are normallyfilled with water for ballast for only a portion of their total time,and they cannot be cathodically protected while empty or while carryingnon aqueous cargo.

While corrosion and cargo contamination of tanks of tanker ships can berestricted by coating all of the interior walls and other exposed metalsurfaces in a tank, with a protective, continuous coating that isresistant to both salt water and hydrocarbon cargos, this is relativelyexpensive, costing normally from one to two dollars per square foot ofsurface. A combination of an inert, corrosion resistant coating coveringall bare surface in contact with water in conjunction with cathodicprotection has been proposed for preserving relatively small and smoothsided tanks such as domestic hot water tanks. In such tanks the cathodicprotective current is very small and is used to prevent corrosion ofminor areas, such as openings or imperfections in the coating.

In relatively large tanks, such as ship cargo tanks, the most common andmost economical method of corrosion protection is the cathodicprotection system normally provided by sacrificial anodes of magnesiummetal. Such systems are designed to protect all or nearly all of thesteel of the tank structure. Single, large, centrally located anodes onthe bottoms within ttheselarge tanks were found to provide an excessiveprotective current to nearby surface areas and inadequate protectivecurrent to areas of the tank surface that are relatively distant fromthe anodes. Adequate cathodic protection for large tanks required amultiplicity of anodes well distributed over all of the tank wall areas.In some of such tanks small di electric shields were used to preventwastage of current through the short paths to nearby met-a1 surfaceareas, but they were only partially successful in achieving uniformcurrent distribution. As a consequence uniform, adequate protectionwithout waste of cathodic material was not achieved.

The distribution of such anodes in large tanks presented difiiculty ininstalling anodes and replacing worn anodes.

Anodic metals, such as aluminum and magnesium, are

produced by an anode fall of two or more feet.

both known to produce hot sparks upon striking a clean steel surface inthe presence of a small amount of iron oxide rust, which action is knownas a Thermit reaction. Under proper conditions it can occur with animpact While sacrificial anodes may be securely'attached initially, theytend to be consumed more rapidly at areas adjacent to their metalsupports, and if this deterioration is allowed to continue to neardepletion, the worn anodes are easily dislodged and may fall.

This hazard is now recognized, and while not eliminated it is alleviatedby replacement of anodes while 'considerable anode metal remains.Substitution of inert anodes will not eliminate the hazard because theymay be dislodged by vibration or agitation and in themselves may causesparks upon striking either tank surfaces or a relatively weak structuresuch as a steam heating coil which then in turn may, in moving, causesparks by striking the steel bottom or other wall. Aside from mechanicaldamage caused by falling anode material, resultant sparks in tanksrecently emptied of inflammable cargos are liable to produce disastrousexplosions.

It is recognized that when liquid cargo tanks are empty or carryingcargo, they normally contain small amounts of water on their bottoms,which results, in the absence of protection, in a severe corrosiveaction commonly called bottom pitting.

An object of this invention is to provide an improved method of andconstruction for providing cathodic protection of metallic containersfor liquids against the corrosion and damage due in part to galvaniccurrents in said container in contact with the liquids therein, whichwill avoid dangers of explosions and damage due to falling anodes, whichwill be exceptionally effective and durable in the protection, easilypracticed, and relatively simple and inexpensive to practice.

Another object is to provide protection to a container for liquids whichwill have adequate resistance to damage due to corrosion and pitting inthe container when partially filled with liquids not in contact withsaid at least one anode, which will have adequate permanence andeffectiveness, which will be resistant to different liquids placed inthe container, and which will be relatively simple, practical, durableand inexpensive in construction.

A further object is to provide for the protection of tanks of cargoships or tankers that may be filled with sea water or liquid cargo asoccasion may require, which provides greater protection than is possibleby cathodic protection alone and at lower cost, which will requirelittle or no servicingattention for relatively long periods, which maybe provided in a container without the necessity of erecting staging andwithout ditficult working conditions, and which will give adequateprotection with a minimum amount of anodic material and at relativelylow cost.

Other objects and advantages will appear from the following descriptionof an example of the invention and the novel features will beparticularly pointed out hereinafter in connection with the appendedclaims.

In the accompanying drawing:

Fig. 1 is a schematic sectional view of .a metal tank of general utilityillustrating one manner in which oath-v odic protection therefore may beprovided in accordance with this invention;

Fig. 2 illustratesschematically a cross section of a simple example of aships cargo tank also provided with cathodic protection in accordancewith this invention; and

Fig. 3 is a schematic circuit diagram illustrating the manner in whichan impressed voltage may be applied when using relatively inert anodes,and also illustrating another manner of mounting anodes of any type in aIn the example of the invention illustrated in Fig. 1, .as applied toany metallic, liquid storage tank 1, such as an industrial storage tank,one or more relatively large bodies 2 of sacrificial anodic material,such as of mag- ,nesium zinc, aluminum, and suitablealloys aremounted inthe interior of a tank .to be protected, preferably near but above andspaced from the bottom 3 of the tank, .and in electrical connection withthe tank. Such anodic bodies 2 are mounted within the lower part of thetank on metallic members' i upstanding from the bottom 3 of the tank, orto any stiffening or reinforcing-members of the tank, or upon anyconvenient projecting structure of the tank, through which anodes areelectrically connected to the tank structure. Preferably-the .anodicbody or bodies 2 should be spaced .Well from the sides of the tank.

A substantially water impervious and continuous coating 6 of adielectric insulating material that is resistant to deterioration bygalvanic currents, water and any other liquid that may be placed 'inthe' tank, such as liquid petroleum products for example, is applied toexposed metal surfaces within the bottom portion of the tank, includingthe floor and any heating and other pipes 7 and .8 and any metal braces,stiffening members and metallic proj'ectionsthat are closely adjacent tothe anode surfaces or to the floor of the tank. This coating issubstantially continuous beneath and in proximity to the anodes and fora substantial distance horizontally therefrom, and it preferably coversthe entire floor and'up the exposed sides of the tank for a substantialdistance, but not exceeding about 50% of the full height of the sideWalls. For example, the coating may advantageously extend up the sidesof the tank from the floor thereof for a distance conveniently reachedby a person While standing on the tank bottom structure, which wouldusually be for a distance from about four to six feet above the part ofthe tank bottom structure on which the person may be standing. Notemporary scaffolding or staging is required. When the side walls arecoated part way up, preferably all exposed surfaces of metal structureswithin the tank bulow the upper 'level of the coating on the tank sidewalls are covered with the dielectric coating material. It is recognizedthat the longer an electrolytic path, the-more uniform is the galvaniccurrent distribution, and in accordance with this invention, there is arelatively long path from the anodic bodies 2 to the nearest exposedmetal surface of any part of the tank ormetallic members or projectionswithin the tank. The minimum length of such current paths will dependsomewhat upon the size of the tank. The minimum length should be largeenough to generally equalize the resistance of the current paths in alldirections from the anodic material to the exposed metal of the tankthat may be in contact with an aqueous liquid held in the tank, but itwill be obvious that all of the possible current paths cannot be closelyequalized, nor is it necessary. It has been determined from actualmeasurements that the resistance of a current path increaseslogarithmically with distance between anode and cathode. The resistancevalue changes very slowly, almost negligibly as the anode to cathodedistance varies above 50 anode diameters, where the anode diameter isdefined as the diameter of a circle Whose area is equal to thecross-sectional area of the anode in question.

It has been found experimentally that major changes in the resistancepath of the liquid occur'within about the first ten said anode diametersdistance between the anode and the cathode, after which the changes arerelatively small. Therefore, for purposes of this invention the minimumanode to cathode distance should be in the order of about 10 said anodediameters measured as the shortest path between the nearest edge of theanode aiildthe bare metallic container cathode. For example, it wouldnot be advisable to locate an anode of 6 equivalent diameter within aminimum of about five feet of any exposed metal in the cargo tanks of anaverage sized tanker ship and a greater distance would be advisable.

In smaller tanks, where a smaller anode may be used, this minimumdistance may be reduced proportionally. This is particularly suited togive adequate uniformity in the galvanic current distribution, and hencemaximum protection to the tank, especially when the tank is filled withsea water.

Where the tank is rather wide or broad and the anodes 2; are arrangednear the central area of the tank bottom, one can obtain a relativelylong path for travel of a galvanic current from the anodes to thenearest exposed metal of the tank when the coating on the bottom wall ofthe tank terminates somewhat short of the side walls and does not extendup the side walls of the tank. Some beneficial results may be obtainedunder such conditions. However, to be certain that adequate cathodicprotection is provided under all conditions, it is preferable to extendthe protective coating close to, or entirely to the side walls, and alsoup the side walls to the extent explained hereinabove, When the tank isemptyor carrying cargo there'is a tendency to severe pitting at thebottom of the tank from small amounts of water that enter in variousways, or which remain whenthe tank is emptied ofwater. The bottomcoating protects against this type of corrosion.

When the tank is filled with water, particularly sea water, the coating6 serves as an insulator to prevent excessive wastage of protectivecurrent to nearby exposed metal of the tank, and automatically causesthe protective current to be distributed with relative uniformity to alluncoated parts of the tank. The coating of the bottom-of the tank, thelower metal parts in the tank and the sides of the tank near the bottomserves also to prevent corrision of those parts While the tank isrelatively empty or carrying non-aqueous cargo. The anodes are locatedto give generous spacing without placing them in close proximity to thetank Walls. The tank dimensions must be considered in this respect.

In applications of this invention to tanks that may ever be expected tocarry combustible or inflammable cargos, the coatings 6 and 20 should bethick and tough enough to resist or prevent penetration thereof byfalling anode material in the tank, and also soft and resilient enoughto avoid the creation of sparks if struck by such falling anodematerial. A suitable coating material for the tank bottom, sides andexposed metal surfaces, is Tarset made by Pittsburgh Coke and ChemicalCompany and which is a coal tar modified, amine cured, epoxy type ofresinous paint. The epoxy type of resin is one formed by a combinationof hisphenol and epichlorohydrin, modified, when desired, by anothermaterial such as coal tar, and hardened by reaction With an amine,suchas diethylene triamine, for example. Another suitable hardener for theresin is available in the trade under the name Versamid 125 made byGeneral Mills, Inc., and which is a fatty acid converted to a long chainamide. The resin may be thinned, when necessary or desirable, by asuitable solvent to facilitate its application to the metal surface.There are a number of variations of the epoxy and ethoxyline type resinsavailable in the market and all of .them appear to be particularlyuseful for coatings in accordance with this invention. Disclosures of asubstantial' number of such resins are given in US. Patents #2,69l,007,#2,324,483, #2,444,333, #2,494,295, #2,500,600, #2,739,l34 and#2,5l1,913, and in British Patents #518,057 and #579,698 to whichreference is made for more information as to such resins. These epoxyand ethoxyline resins are particularly useful as the coatings 6 and 20because they can form dielectric films or coatingsrof any desiredthickness that are sufficiently soft, resilient and tough to resist andprevent penetration thereof, and'formation of sparks by anodic materialfalling thereon. Another type of coating that may be used is zincsilicate or a cementitious material such as Portland cement mixed withan added silicate to make the coating impervious to liquids.

Additional resinous coating materials that may be used are theisocyanate resins, and the aldehyde condensation type of resins ofeither the cold cured or the hot set type. A useful coating in theabsence of organic solvents in the tanks may comprise dielectricmaterials having elastomeric properties, such as tar, latex, syntheticrubber, asphalt, and mixtures thereof. Other coating materials may beused if they have the desired properties of being dielectric, soft,tough and'resilient, and resistant to galvanic currents, water and otherliquids that may be placed in the tank to be protected. For use intanker ships, the coating should also be resistant to organic liquid cargos as well as salt water.

Referring next to the example of the invention illustrated in Fig. 2,the tank protected is shown schematically as a tank or holder such asmay be found in a typical cargo or petroleum tanker ship. It has abottom wall 9, side walls 10, a top 11, spaced ribs 12. upstanding fromthe bottom, braced by spaced apart cross plates 13 with apertures 14,and cross bars 15, all of metal. Pipes 16 extend above the bottombetween the ribs 12 and pass through apertures in the cross plates 13.The anodes 17 are mounted on the ribs 12 and bars :15 by short metaluprights 18 that provide electrical connection from the anodes to thetank walls. The centralanode 17 may be, if desired, disposed or extendedslightly higher than the anodes nearer the side walls. The side walls ofthe tank may have stiffening strips 19 along the sides near but abovethe bottom wall 9 of the tank. A coating 20, similar to coating 6 ofFig. l, is applied to the exposed upper surface of the bottom Wall 9,the exposed surfaces of the stiifening and bracing members 12, 13, 15and 19, the piping 16, the anode supports 18 where exposed in the lowerpart of the tank, and also preferably the inner exposed sides of thetank for a short distance up from the bottom, as explained in connectionwith tank 1 of Fig. 1.

Where the tank of Fig. 2 is broad in horizontal dimensions and theanodes are space well from the sides of the tank, the bottom wall iscoated in the areas beneath the anodes and horizontally therefrom for anarea against which the anode may fall, and approaching the side walls atleast for enough to provide a path of minimum length of ten said anodediameters from the anodes to the exposed metal surfaces within the lowerpart of the tank, but in such cases the coating may not reach themargins of the bottom wall or extend up the sides. However, the coatingpreferably extends over substantially all of the bottom and up the sidewalls and over the metal parts in the lower part of the tank asexplained for Fig. 1.

It is possible to use for protection, in all embodiments of theinvention, non-sacrificial or relatively inert anodes 2 or 17, when asmall, impressed, direct curent voltage is applied between the anodesand the metal of the tank. In such cases the anodes may,'for example, beof metallic lead, high-silicon iron, platinum or graphite, and theimpressed current may be obtained from a generator, batteries or arectifier. While any impressed voltage may be applied, it is advisableto use only small voltages of about 2 to 6 volts for safety andeconomical reasons. A dielectric coating 6 or 20 should be applied asexplained hereinabove for Figs. 1 and 2. This is illustratedschematically in Fig. 3 in which relativelyinert anodes 21 are mountedin any suitable manner in the tank 22 but insulated from the tank bytheir supports 23. The anodes are electrically connected by insulatedconductors 24 to the positive side of a battery 25 or other source oflow DC. voltage, the negative side of the battery being connected byconductor 26 to the metal of the tank.

In some instances, particularly in stationary tanks, it may be desirableto support an anode from the top of the tank, but with the anode inproximity to the bottom, which makes it easy to remove the anode fromthe tank for inspection or replacement. This is also illustrated in Fig.3 in which an anode 27 is suspended by a conductor 28 from the top ofthe tank 22. The conductor 28 should be covered with an insultingcoating, or provided with such a coating at the time of its manufacture.The conductor 28 is electrically connected to the anode 27, and if thisanode is of the sacrificial type the conductor 28 will be connected tothe tank. If the anode is of the relatively inert type, then theconductor 28 should be connected by a wire 29 to the positive side ofthe impressed voltage from DC. source or battery 25. The suspendedanodes 27 may be used alone or in conjunction with the anodes 21 thatare supported from the bottom of the tank.

The present cost of providing cathodic protection in an average tank ofa T-2 type tanker ship is about $2,500 where numerous magnesium anodesare placed all around in the tank. Replacement costs of about $1,000 arenecessary about every two years so that at the end of twelve years, thetotal cost is about $7,500. With the arrangement of anodes and coatingin accordance with this invention, cathodic protection for twelve yearswill cost about $5,200 for the same size tank, in which the initialinstallation cost is about $4,000, with anode replacement and paintrepairs at four year intervals costing about $600 each. This lower costfor a long range installation in accordance with this invention ispossible because in the case of a ship tank, all of the installationwork is performed at the bottom of the tank without the use of anyerected staging or scaffolding and without difiicult working conditions.Furthermore, in order to protect an unpainted area of about 70% of thetank, 70% or less of the magnesium is required and this may beconcentrated in larger, more economical sizes such as 70 lb. magnesiumanodes instead of the more commonly used 50 lb. magnesium anodes. Whenthe larger anodes are used, fewer need be installed.

The service life of the anodes is approximately doubled in accordancewith this invention because of the saving of the large amount of currentwasted at low resistance cathode areas near the anodes. The size andamount of galvanic anodes will vary for each tank geometry and internalsurface area, but generally sufficient anode area and weight areprovided to produce a total current output in the range of about 3 to 10milliamperes per square foot of exposed cathodic tank surface for abouta two year period. For intermittent usage such as salt water ballastingcargo oil tanks of tankers, the galvanic anodes will last about twice aslong. [For impressed current inert anodes, the size and shape of theanode is determined by the current output required and the drivingvoltage specified.

While the invention has been described as used in connection with tanksof ships and other tanks into which heating pipes extend as examples ofuse of the invention, the same protection is useful for, and may beadvantageously employed with, any large metal tanks for continuous orintermittent storing or transporting of water, or other corrosiveliquids. The heating pipe could be omitted when no heating of thecontents of the tank is desired.

This application is a continuation-in-part of our copending applicationSerial No. 697,504, filed November 19, 1957.

It will also be understood that various changes in the details, steps,materials and arrangements of parts, which have been herein describedand illustrated in order toexplain the nature of the invention, may bemade by those skilled in the art within the principle and scope of theinvention as expressed in the appended claims.

We claim:

1. A metallic holder for liquids having uniform, efiicient cathodicprotection against corrosion, by any liquids contained therein, of itsexposed interior surfaces, which comprises a metallic enclosure for saidliquids of the size of cargo tanks of ships, and having electrolyticanodic material supported by said enclosure within it and spacedsomewhat above but in proximity to the floor of the enclosure or anyprojecting structure just above the floor, said enclosure havingmaterial on that portion of its floor and any said projecting structurewhich lies beneath said anodic material and horizontally therefrom overany area upon which any of said anodic material might fall if dislodgedfrom its support, a coating of a dielectric material that is inert andimpervious to any liquids likely to be placed in said enclosure andwhich when struck by any metal falling through a distance equal to thedepth of the enclosure will not create sparks and leaving at least 40%of the inner surface of the on closure uncoated and exposed to saidliquids that may be placed in said enclosure, and circuitry connectingsaid anodic material and said enclosure to form therewith anelectrochemical cell when any liquid placed in said enclosure functionsas an electrolyte.

2. The holder as setforth in claim 1, wherein the minimum directdistance between said anodic material and the nearest uncoated, exposedmetal surface within said enclosure is at least about ten anodediameters where an anode diameter is defined as the diameter of a circlehaving an area equal to the cross-sectional area of a body of saidanodic material within the enclosure, said minimum distance representingthe minimum length of current path between said anodic material and theexposed uncoated metal of said enclosure required for substantiallyuniform current distribution between said anodic material and the metalof said enclosure through the liquid contained within the enclosure.

3. The holder as set forth in claim 1, wherein said coating coverssubstantially the entire floor of said enclosure and any of theprojecting metallic structure of the enclosure which is attached to saidfloor and lies below the highest level of any of said anodic material.

4. The holder as set forth in claim 1, wherein said coating also extendsup the side walls of the enclosure from the floor for a minor part ofthe height of such side walls, the balance of the side walls beingsubstantially and directly exposed to any liquid filling the enclosure.

5. The holder as set forth in claim 1, wherein said coating has athickness and strength adequate to prevent its penetration by any ofsaid anodic material which may fall thereon.

6. The holder as set forth in claim 1, wherein said anodic material isinthe form of at least one anode body, and its support by the enclosureis through a metal member that electrically connects the body andenclosure.

7. The holder as set forth in claim 1, wherein said coating is basicallycomposed of an amine cured, epoxy type of resin.

8. The holder as set forth in claim 1, wherein said coating is basicallycomposed of a long chain amide cured, epoxy type of resin.

9. The holder as set forth in claim 1, wherein said coating is largelyzinc silicate.

10. The holder as set forth in claim 1, wherein said coating comprisesan isocyanate resin.

11. The holder as set forth in claim 1, wherein said coating comprisesan aldehyde condensation type of resin.

12. The holder as set forth in claim 1, wherein said coating contains asubstantial amount of coal tar.

13. The holder as set forth in claim 1, wherein said coating haselastomeric properties.

14. The holder as set forth in claim 1, wherein said anodic'material isof the sacrificial type.

15. The holder as set forth in claim 1, wherein said anodic materialincludes an anode body carried by the upper part of said enclosure by aninsulated, electrically conductive element.

16. Holders as set forth in claim 1, wherein said anodic material ineach enclosure is in the form of at least one body and said bodies areselected in size, number and positions in an enclosure to produce ineach enclosure a total concurrent galvanic current output in the rangeof about three to ten milliampercs per square foot of exposed, uncoatedmetal surfaces within said enclosure.

17. A holder as set forth in claim 1, wherein said anodic material isactivated by a driving voltage sufficient to produce a current of aboutthree to ten milliamperes per square foot of exposed, uncoated metalsurfaces within said enclosure.

18. A metallic holder for liquids having uniform, efficient, cathodicprotection against corrosion, by any liquids contained therein, of itsexposed interior surfaces, which comprises a metallic enclosure for saidliquids having a size at least as large as cargo tanks of ships,electrolytic anodic material supported within and by said enclosure andspaced somewhat above but in proximity to the floor of the enclosure andof any projecting structure just above said lioor on an interior face ofsaid enclosure, said enclosure having a continuous soft coating of adielectric material solely on that part of the inside face of saidenclosure on that portion of its floor and anyof said projectingstructure which lies beneath said anodic "material, and horizontallyfrom said anodic mate-' rial over any area upon which said anodicmaterial might fall if it became released from the part of the enclosureto which it was attached for support and for a distance from said anodicmaterial adequate to cause substantially uniform current distributionbetween said anodic material and any uncoated exposed metal of theenclosure through any liquid contained within said en closure, saidcoating material being impervious and inert to any liquid likely to beplaced in the enclosure, and covering less than about 60% of theinterior wall surface of the enclosure, the balance of such interiorwall surface being uncoated and exposed to direct contact with thecontents of the enclosure.

References Cited in the file of this patent UNITED STATES PATENTS1,512,557 Mills Oct. 21, 1924 2,329,961 Walker Sept. 21, 1943 2,459,123'Bates et al. Jan. 11, 1949

1. A METALLIC HOLDER FOR LIQUIDS HAVING UNIFORM, EFFICIENT CATHODICPROTECTION AGAINST CORROSION, BY ANY LIQUIDS CONTAINED THEREIN, OF ITSEXPOSED INTERIOR SURFACES, WHICH COMPRISES A METALLIC ENCLOSURE FOR SAIDLIQUIDS OF THE SIZE OF CARGO TANKS OF SHIPS, AND HAVING ELECTROLYTICANODIC MATERIAL SUPPORTED BY SAID ENCLOSURE WITHIN IT AND SPACEDSOMEWHAT ABOVE BUT IN PROXIMITY TO THE FLOOR OF THE ENCLOSURE OR ANYPROJECTING STRUCTURE JUST ABOVE THE FLOOR, SAID ENCLOSURE HAVINGMATERIAL ON THAT PORTION OF ITS FLOOR AND ANY SAID PROJECTING STRUCTUREWHICH LIES BENEATH SAID ANODIC MATERIAL AND HORIZONTALLY THEREFROM OVERANY AREA UPON WHICH ANY OF SAID ANODIC MATERIAL MIGHT FALL IF DISLODGEDFROM ITS SUPPORT, A COATING OF A DIELECTRIC MATERIAL THAT IS INERT ANDIMPERVIOUS TO ANY LIQUIDS LIKELY TO BE PLACED IN SAID ENCLOSURE ANDWHICH WHEN STRUCK BY ANY METAL FALLING THROUGH A DISTANCE EQUAL TO THEDEPTH OF THE ENCLOSURE WILL NOT CREATE SPARKS AND LEAVING AT LEAST 40%OF THE INNER SURFACE OF THE ENCLOSURE UNCOATED AND EXPOSED TO SAIDLIQUIDS THAT MAY BE PLACED IN SAID ENCLOSURE, AND CIRCUITRY CONNECTINGSAID ANODIC MATERIAL AND SAID ENCLOSURE TO FORM THEREWITH ANELECTROCHEMICAL CELL WHEN ANY LIQUID PLACED IN SAID ENCLOSURE FUNCTIONSAS AN ELECTROLYTE